1. Field of the Invention
The present invention relates to an eddy current flaw detecting probe used for an eddy current test to determine an internal flaw non-destructively.
2. Description of the Prior Art
An eddy current test is now widely practiced for tests in manufacturing iron and steel material and non-ferrous metal material, and maintenance inspection tests in various plants including small diameter tubes for heat exchangers and the like. An eddy current flaw detecting probe is one of the important factors which decide performance of a flaw detector.
One example of a prior art eddy current flaw detecting probe is shown in FIG. 11. Numeral 1 designates a test object, numeral 50 designates an exciting and detecting coil, numeral 51 designates an exciting coil and numeral 52 designates a detecting coil. In the prior art eddy current flaw detecting probe, there are used a bobbin coil, a pancake coil and the like as the exciting detecting coil. The flaw detecting probe is divided into an absolute type [FIGS. 11(a) and (b)] for testing for the presence of a flaw by impedance change in the respective coils 50, 52 and a differential type [FIGS. 11(c), (d) and (e)] for the presence of a flaw by differential component in the two coils 50, 52.
Also, the flaw detecting probe is divided into a self-induction type [FIGS. 11(a), (c) and (e)], in which the same coil 50 carries out both excitation for inducing eddy current and detection of magnetic field by the eddy current, and a mutual induction type [FIGS. 11(b) and (d)] which comprises two kinds of coil including the exciting coil 51 for excitation and the detecting coil 52 for detection.
The differential type, especially, has an advantage (as compared with the absolute type) in dealing with noises caused by horizontal lift-offs in which a distance between the coil and the test object changes. In the prior art eddy current flaw detecting probe (especially of the absolute type), lift-off signals due to lift-offs occur so that a flaw signal may be buried, which results in problems such as a reduction in detecting power.
Further, even in the differential type which is during generally good for lift-offs, during an inclined lift-off (FIG. 12) in which the probe inclines relative to the test object, there occurs a difference in distance 1.sub.1, and 1.sub.2 from the two coils to the test object 1. This difference causes inclined lift-off signals, which result in problems such as a reduction in the flaw detecting power. It is to be noted that numeral 51 designates an exciting coil.
Also, in the eddy current flaw detecting probe shown in FIG. 11(e), there is less reduction in the detecting power against the inclined lift-off. This probe is constructed such that two coils, arranged so as to cross each other, are mutually in a differential connection. This probe has, however, a directivity in the detecting power and there is a shortcoming that it has especially less flaw detecting power in the angle of 45.degree. to a scanning direction.